Conventionally, metal substrates for industrial use are given in the course of surface preparation a zinc phosphate treatment for the purpose of improving corrosion resistance or adherability. However, zinc phosphate treating agent used in the chemical treatment contains large quantities of phosphorus or nitrogen and also contains large quantities of heavy metals such as nickel and manganese for improving the performance of the formed chemical coating, which gives rise to such problems as adverse influences on environments and disposal of industrial waste because the treatment generates a large amount of sludge of zinc phosphate, iron phosphate and the like.
Also for the purpose of improving corrosion resistance of industrial metal substrates, much space and time are required for coating lines for such processing steps as “degreasing-surface treatment-chemical treatment-electrodeposition coating”.
JP 2003-155578A proposed a chemical treating agent for iron- and/or zinc-based substrates, which contains substantially no phosphate ion but contains zirconium ion and/or titanium ion and fluorine ion. However, the chemical treating agent for iron- and/or zinc-based substrates as described in JP 2003-155578A has a problem in that satisfactory corrosion resistance or finish cannot be secured unless a coating film is applied thereon by a coating step after the treatment using said agent.
International Publication WO 02/103080 pamphlet discloses a technology for reducing the time and space required for the treating steps by the use of a composition for metal surface treatment, which comprises (A) a compound containing at least one metal element selected from Ti, Zr, Hf and Si and (B) a fluorine-containing compound as a supply source of fluorine ion, whereby precipitating a surface treating film excelling in corrosion resistance on a metal surface containing at least either of iron or zinc, and dispensing with a surface adjustment (leveling) step. This surface treating composition disclosed in International Publication WO 02/05860 pamphlet, however, is also subject to the problem of failing to secure satisfactory corrosion resistance or finish, unless a coating film is applied thereon by a coating step after the treatment therewith.
JP 2003-166073A and JP 2003-226982A disclose a surface treating agent for lubricated steel sheet, which contains (A) amine-modified acrylic resin, (B) at least one compound selected from phosphoric acid-derived compounds, hydrofluoric acid, metal hydrofluoric acid and metal hydrofluoric acid salt, and (C) at least one compound selected from molybdenum compound, tungsten compound and vanadium compound; and which, when coated on zinc-plated steel sheet useful for automobile bodies or household electric appliances, can provide lubricated steel sheet excelling in press-shapability and corrosion resistance. However, the steel sheet which is surface treated with the surface treating agent as disclosed in JP 2003-166073A or JP 2003-226982A fails to show satisfactory corrosion resistance or finish unless a coating film is applied thereon by a coating step after the chemical treatment, and the invention cannot achieve reduction in steps or space-saving.
JP 2003-293161A discloses a polymer composition for metal surface treating agent, which comprises a specific copolymer having salicylideneamino group and amino group. The steel sheet treated with the polymer composition for metal surface treating agent as described in JP 2003-293161A again fails to show satisfactory corrosion resistance or finish, unless a coating film is applied thereon by a coating step, and the invention cannot lead to reduction in steps or space-saving.
Furthermore, JP Hei 2(1990)-282499A discloses a method for forming a coating film on apertures of coating object having complex construction such as an automobile body having apertures of not more than 500 μm in width, by cationic electrodeposition coating according to multistage electricity applying method. The method as described in JP Hei 2(1990)-282499A is effective for improving corrosion resistance of a coating object having apertures of not more than 500 μm in width, by coating the apertures, but does not amount to secure satisfactory corrosion resistance or finish.
JP 2003-328192A (EP1342758A) discloses a method for forming multilayer electrodeposition coating film by applying a cationic electrodeposition paint containing plural emulsions among which the differences in quantity of electricity necessary for starting precipitation are unified. This method, however, is yet incapable of providing sufficient corrosion resistance.